My 2 cents - you say push button - I have personally fixed two of these where the switch wasn't faulty it was the funny red plastic cranked lever between the button you push and the switch itself that was busted. This crank is itself held by two red plastic mounts that are visible at the front of the base - certainly on early Mag-Alloy cased Cyrus 1/2. The plastic mounts break and the switch stops working.

Ring up Cyrus and they will post a couple of replacement clips for free - don't you just love dealing with a company that believes in Service.

The font of the case comes off with 2 screws at the bottom and 2 torx screws at the top sides plus the knobs removed and then you poke out the old broken red clip from the inside outwards and push in the new replacement clip from the outside inwards - 2 mins work.

There is also a toggle switch that was used on later models - can't fix this, need to replace. I recall seeing something that seemed to have the correct dimensions in the CPC/Farnell catalogue - must get around to buying a couple.